1. Field of the Invention
This invention generally relates to systems and methods for determining a characteristic of a specimen. Certain embodiments relate to a system configured to direct light to a first set of spots on a specimen at a normal angle of incidence and to simultaneously direct light to a second set of spots on the specimen at an oblique angle of incidence and to detect light scattered from the first and second sets of spots simultaneously.
2. Description of the Related Art
The following description and examples are not admitted to be prior art by virtue of their inclusion in this section.
Fabricating semiconductor devices such as logic and memory devices typically includes processing a substrate such as a semiconductor wafer using a large number of semiconductor fabrication processes to form various features and multiple levels of the semiconductor devices. For example, lithography is a semiconductor fabrication process that involves transferring a pattern from a reticle to a resist arranged on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing, etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated in an arrangement on a single semiconductor wafer and then separated into individual semiconductor devices.
Inspection processes are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield in the manufacturing process and thus higher profits. Inspection has always been an important part of fabricating semiconductor devices such as integrated circuits. However, as the dimensions of semiconductor devices decrease, inspection becomes even more important to the successful manufacture of acceptable semiconductor devices because smaller defects can cause the device to fail. For instance, as the dimensions of semiconductor devices decrease, detection of defects of decreasing size has become necessary since even relatively small defects may cause unwanted aberrations in the semiconductor devices. Accordingly, much work has been done in the field of wafer inspection to increase the sensitivity of inspection systems to smaller and smaller defects.
In unpatterned or monitor wafer inspection, it is known that, on average, smaller defects can be detected at higher throughput by using light incident on the wafer at an oblique angle from the surface normal, as compared to normal illumination. However, normal incidence illumination has advantages over oblique incidence illumination in other respects. For instance, normal incidence illumination can yield uniform, and sometimes better, detection of scratches, micro-scratches, slip-lines, planar features, and other asymmetric defects of interest.
One way to obtain maximum information from a single wafer inspection includes illuminating the wafer simultaneously at both normal and oblique angles of incidence and collecting surface scatter information from both modalities at once. In addition to simultaneous illumination and collection, the defect detection, processing, and classification subsystems must be able to separate or combine scattered light signals as a function of illumination angle. In this manner, the information obtained using normal incidence illumination and oblique incidence illumination must be separately preserved. Typically, however, using unpatterned surface inspection systems, sequential scans with obliquely incident light and normally incident light are performed to obtain the best capture and classification of surface anomalies and defects. While convenient, this approach does not maximize speed of inspection.
Another advocated approach is to use two inspection wavelengths simultaneously; one wavelength obliquely incident on the wafer, and the second wavelength normally incident on the wafer. In this way, the light collection subsystem may use a physical apparatus, for instance, thin-film filters, to separate the light scattered as a function of incidence angle into separate detectors. Then, two or more detection signals can be generated and sent to the processing subsystem. This approach offers higher throughput than the sequential scan approach, but suffers from the increased cost of additional optical hardware, as well as using two different wavelengths for the inspection. Occasionally, defects or anomalies may have relatively large dispersion (e.g., their scattering properties may depend dramatically on the illuminating wavelength). In these cases, using multiple wavelengths for inspection can be beneficial, and one can argue in favor of dual wavelength detection and classification. But in other cases, a single wavelength is preferred. For instance, the defect detection threshold of the longer of the two wavelengths selected for inspection may be larger in terms of size than that of the shorter wavelength, and the longer wavelength inspection could miss defects.
A third approach is to use the same wavelength simultaneously for illumination at both normal and oblique angles of incidence and to separate inspection at the normal and oblique angles of incidence in either the time domain or in the spatial domain. Time domain separation demands challenging high speed synchrony of the illumination, collection, and data processing subsystems. On the other hand, spatial separation demands that the scattered light collection subsystem direct light scattered from the normally incident illumination to one or more detectors and the light scattered from the obliquely incident illumination to one or more detectors. Currently available inspection systems such as the SP1 and SP2 tools, which are commercially available from KLA-Tencor, San Jose, Calif., have a non-imaging scattered light collector (e.g., a section of an ellipsoidal mirror), and this collector does not permit such spatial separation.
Accordingly, it would be advantageous to develop systems and methods for determining a characteristic of a specimen such as a presence of defects on the specimen that can detect relatively small defects and a relatively large variety of defects with relatively high throughput by illuminating the specimen with one or more wavelengths of light at both normal and oblique angles of incidence and detecting light scattered from the specimen as a function of illumination angle without substantially increasing the complexity or cost of the systems and methods.